1. Field of the Invention
The invention relates to an optical fiber cable and more specifically, an optical fiber cable used for laying in premises.
2. Description of Related Art
Layer type optical fiber cable and loose tube distribution cable are known in the prior art as an optical fiber cable having an indispensable rigid tensile strength member, used for installation in premises.
As shown in FIG. 1, the layer type optical fiber cable 301 is constructed having a rigid central strength member 303 of steel wire or FRP (Fiber Reinforced Plastic) rod disposed in the center thereof, a plurality of optical fibers 305, disposed around the rigid central strength member 303, stranded either in one direction or in alternating directions (i.e. SZ), a wrap 307 disposed around the plurality of optical fibers 305, and a sheath 309 covering the wrap 307.
As shown in FIG. 2, the loose tube distribution cable 311 is constructed having tensile strength filaments 313 helically around a bundle of optical fibers 305 and a sheath 315 covering the tensile strength filaments 313. Because the loose tube distribution cable 311 does not have a rigid central strength member in the center part thereof, the cable 311 has an advantage in comparison to the cable 301, of bending more easily when installed. Further, as material of substantial specific gravity such as steel wire or FRP rod or the like is not used in the loose tube distribution cable 311 (as it is in the layer type optical fiber cable 301), the loose tube distribution cable 311 is lighter. Loose tube distribution cables 311 are disclosed in, for example, Japanese Unexamined Patent Publication No. Hei 11-183764 and Japanese Unexamined Patent Publication No. 2003-5002.
A problem affecting the layer type optical fiber cable 301 of the related art is that, as steel wire or FRP rod having a high modulus of rigidity is used for a central strength member, it is difficult to bend the optical fiber cable itself. Another problem affecting the layer type optical fiber cable 301 is that the optical fibers 305 are pressed against the rigid central strength member 303 in response to lateral pressure or compression, causing localized bending that increases transmission loss of the optical fiber.
The loose tube distribution cable 311 is improved with respect to the weaknesses inherent in the layer type optical fiber cable 301. However, when lateral pressure or compression is applied to the cable 311, optical fibers 305 therein are apt to cross over each other or tightly contact each other, resulting in an increase in transmission loss.